This invention relates to direct current, permanent magnet motors, and more particularly to a motor with an improved field pole which substantially reduces cogging.
Direct current permanent magnet motors are used in a wide variety of applications including servo motor and other control applications. Such motors typically have two or more permanent magnet poles to create a magnet field and an armature rotatably mounted in the field. The armature has a series of bars or teeth evenly spaced around its periphery. Conductors wound in slots formed between the bars carry the energizing current for the motor. Usually the armature of such motors is constructed from a series of discs or laminas stacked on a shaft.
As those skilled in the art will appreciate, the magnetic attraction between the permanent magnet pole pieces and the armature bars vary as the bar moves into proximity with the edge of the pole piece. This variable attraction produces an effect called cogging.
Cogging has two deleterious manifestations. One is a variable torque characteristic, and the other is a preferred stable positional relationship between the armature and the pole piece.
There have been a number of proposals in the prior art for reducing, and or eliminating cogging. One prior art proposal, which is widely used is to skew the armature with respect to the edges of the pole pieces. If the skew is equal to the bar pitch, cogging can be substantially eliminated. The drawbacks of this approach include the fact that motors with skewed armatures are somewhat more difficult to manufacture than those with axially aligned armatures, and the number of conductors which can be fitted into a skewed armature slot is somewhat reduced.
At least two other proposals have been made in the prior art as alternatives to skewing the armature in order to eliminate cogging. One prior art proposal is to shave or cut the edges of the pole piece in order to achieve an effect similar to that achieved by skewing the armature. The other is to skew the pole piece itself. Neither of these proposals has proved altogether satisfactory.
Another approach to eliminate cogging is shown in U.S. Pat. No. 3,604,961. This approach cancels the positive and negative cogging effects generated by the armature bars as they respectively enter and leave the pole piece by locating the pole pieces eccentrically with respect to a plane of symmetry. Again, this approach is not proven altogether satisfactory.
An object of this invention is to provide a permanent magnet motor which does not have a tendency to cog, and can be manufactured economically.
Briefly, this invention contemplates a permanent magnet DC motor which has a series of notches or cutouts along the edges of its permanent magnet field poles. These notches have been found to substantially reduce or eliminate cogging without adding to the cost of the motor.